The lid completely occupies the top, and in a smaller inner circle the micropyle is situated. The latter is better displayed when the lid is separated from the egg, as shown at a. The White butterfly (Pieris brassica), Fig. 13. The shape of the egg is very like the basket employed in lobster fishing, a rarer form than any of the preceding. It is conical, and of considerable length; the lid forms the base, which is slightly recurved upon the sides, and a regular series of ribs with cross bars run from end to end. The eggs are cemented at the base to the back or leaf of the plant in symmetrical order. In colour they are primrose. The Brown-hair streak butterfly (Thecla betulæ), Fig. 15, presents a perfectly white, exquisitely formed, sub-conical egg; at first sight it might be compared to a beautiful ivory-turned ball in miniature. It is covered by a series of deep indentations, or pits, with regularly projecting spines. The pole of this egg dips inward towards the micropyle, forming the funnelshaped indent spoken of by Leuckart. It is cemented by its broader base to the leaf. I may remark that the specimens used for illustration were not specially selected, nor are they intended to be type representatives of the eggs of a class of insects which constitute a very large proportion of the most charming denizens of our gardens, fields, and forests. These eggs are taken from a very limited collection, and in no way do they convey an adequate notion of the variety and beauty of objects, wonderfully and curiously fashioned, no two of the species of which are to be found exactly alike. My thanks are due to my friend, Mrs. Maples, for the accurate and beautiful plate which her skilful pencil has enabled me to place before my readers. The subscribers to the INTELLECTUAL OBSERVER will be glad to know that they can obtain most of these eggs from Mr. J. T. Norman, of City Road. A LIST OF THE ILLUSTRATIONS. 1.Abraxas grossularia, Magpie moth. 3.-Exarnis ypsilon, the Dingy shears. 4.-Pheosia dictaa, Swallow prominent. 5.-Ennomos erosaria, Thorn moth. 6.--Ourapteryx sambucaria, Swallow tailed. 7.-Diacrisia russula, Buff tiger. 8.-Erannis defoliaria, Mottled umber. 9.-Hylophila prasinana, Silver lines. 10.-Cerura vinula, Puss moth. 17.-Egg of Honey bee, showing germinal vesicle. RAIN. BY RICHARD A. PROCTOR, B.A., F.R.A.S. THERE are, perhaps, few natural phenomena which appear less indicative, at first sight, of the operation of nature's giant forces than the downfall of rain. Even the heaviest showers-at least of those we are familiar with in England-are not phenomena. which suggest an impression of power. Yet the forces actually called into action before rain can fall, are among the most gigantic experienced on our earth. Compared with them, terrestrial gravitation is more feeble than is the puniest infant compared with an array of giants. Let us look into the matter a little closely, and we shall see that this is so. It is a common occurrence for rain to fall over an area of 100 square miles to a depth of one inch in twenty-four hours. Now, what is the expenditure of power of which such a phenomenon is the equivalent? The downfall is, so to speak, the loosening of the spring, but how much force was expended in winding up the spring? The evaporation from the sea or from moist soils of the quantity of water precipitated, is not the whole of the work to be estimated, since the vapour has to be raised to the higher regions of the air, and to be wafted by the winds-themselves the representatives of giant forces-to the district over which the moisture is discharged in rain. But let us take this evaporation only, and estimate its real force-equivalent. It may be shown by a calculation founded on M. Joule's experiments, that to evaporate a quantity of water sufficient to cover an area of 100 miles to a depth of one inch, would require as much heat as is produced by the combustion of half a million tons of coals; and further, that the amount of force of which such a consumption of heat is the equivalent, corresponds to that which would be required to raise a weight of upwards of one thousand millions of tons to a height of one mile! I will run briefly through the calculation by which this last result is deduced from the well-known result of Joule's experiments that to raise one pound of water one degree Fahrenheit, requires a quantity of heat sufficient to raise one pound to a height of 772 feet; and the further experimental fact, that to raise a pound of water from the liquid to the vaporous state, requires 967 times as much heat as is required to raise the same pound one degree Fahrenheit in heat. The amount of water required to cover one hundred square miles to a depth of one inch is, in volume— 1760 x 1760 x 3 x 3 x 100÷12 cubic feet, and as one cubic foot of water weighs 1000 oz., or nearly 63 pounds, we have in weight— 1760 x 1760 x 3x3x8 x 62 pounds, and to raise this weight of water 1° F., would require as much heat as would suffice to raise to a height of one mile a weight of 1760 × 3 × 8 × 62 x 772 pounds; while to vaporize the same weight of water would require 967 times as much heat. Thus we obtain a force sufficient to raise a weight of 1760 x 3 x 17 x 135 x 193 x 967 pounds, (that is, nearly 1,020,000,000 tons), to the height of one mile. Such is the amount of force, whose effects are exhibited in a day's steady down-pour over a region of 100 square miles— for instance, over about one-third of Middlesex. The same amount of water falling in the form of snow, would represent a yet greater expenditure of force." I have seen," says Tyndall, "the wild stone-avalanches of the Alps, which smoke and thunder down the declivities, with a vehemence almost sufficient to stun the observer. I have also seen snow-flakes descending so softly as not to hurt the fragile spangles of which they were composed; yet to produce, from aqueous vapour, a quantity which a child could carry, of that tender material, demands an exertion of energy competent to gather up the shattered blocks of the largest stone-avalanche I have ever seen, and pitch them to twice the height from which they fell." But it is when we come to estimate the fall of rain as a terrestrial phenomenon-as a process continually going on over large regions of the earth's surface, as a process in which energies exhibited over one region are expended, frequently, over regions thousands of miles away-that we see the full significance of the drop of rain. We can well understand how it is that "the clouds drop fatness on the earth," when we estimate the powers expended in their genesis. All the coal which could be raised by man from the earth in thousands of years, would not give out heat enough to produce by evaporation the earth's rain-supply for one single year! The sunwhose influence is often contrasted with that of the rainshower-is the agent in producing that shower as well as in pouring out his direct heat on the soil with such apparently contrasted effect. The actual process of the production of rain has not yet been completely explained. We are, in fact, doubtful as to the true nature of clouds, fogs, and mist, and, therefore, it is intelligible that some difficulty should surround the explanation of a phenomenon of which these meteors are, so to speak, the parents. It is generally supposed that clouds consist mainly of hollow vesicles of water, and not of minute drops. Yet meteorologists are far from being agreed on this point. On the one hand we have the evidence of De Saussure and Kratzenstein, who actually saw, or supposed they saw, the constituent vesicles of clouds and fogs. De Saussure, indeed, tells us how we may see the vesicles for ourselves. If a cup of coffee, or of water tinctured with Indian ink be placed in the sun, minute vesicles of various thickness will be seen to ascend from the surface of the liquid. He adds that those vesicles which rise differ so much in appearance from those which fall, that it is impossible to doubt that the former are hollow. Kämtz, also, made measurements of the vesicles of fogs in Central Germany and in Switzerland; and in his valuable work on Meteorology, gives a table and a figure, showing the law according to which the dimensions of the vesicles vary in the course of the year. Despite this evidence, Sir John Herschel holds a contrary opinion. He points out that the observations of De Saussure and Kratzenstein may be readily referred to the effects of optical illusion. The strongest argument put forward by Kratzenstein is founded on the fact that rainbows are never formed on clouds or fogs, as they would be (according to the undulatory theory of light) if these meteors were composed of globules of water. Sir John Herschel, a higher authority on optical questions than either De Saussure or Kratzenstein, is of opinion, on the contrary, that it is possible a re-examination of the very difficult point in question would give a different account than that usually accepted. Herschel points out the difficulty of understanding in what manner the condensation of true vapour should result in the formation of a hollow vesicle. Tyndall points out, on the other hand, a difficulty depending on the state into which waterparticles at high elevations sometimes pass. "It is certain," he says, "that they possess, on or after precipitation, the power of building themselves into crystalline forms; they thus bring forces into play which we have hitherto been accustomed to regard as molecular, and which could not be ascribed to the aggregates necessary to form vesicles." In whatever state the particles of a cloud really exist, it is certain that the fall of rain depends on a process of increased condensation. The causes producing such condensation have been thus summed up by Professor Nichol : (1.) The cooling of clouds through the effect of radiation from them; (2.) The mingling of vapours at different temperatures-a mingling effected by the agency of the winds; (3.) The rising of vapours towards colder strata of the atmosphere; (4.) The increase of atmospheric density or pressure; And (5.) The accumulation and impinging of masses of vapour against some obstacle. Singularly enough he omits the most important of all known agencies in the production of rain, viz. :— (6.) The transfer from the equator towards the poles of large masses of moisture-laden air by means of the upper S.W., or counter trade-winds. I must note also that cause (4) is more than doubtful. Tyndall has shown that rarefaction is an efficient agent in producing the precipitation of vapour. By increase of pressure a larger quantity of moisture is, indeed, compressed within any given space; but, on the other hand, there is an increase of heat within the space which more than counterbalances the former in effect. The heat developed," says Tyndall, speaking of an experiment illustrating the effects of increased pressure, "is more than sufficient to preserve it" (the moisture added to a given space) " in the state of vapour.' It will be seen at once from the above imperfect enumeration of causes affecting the production of rain, that the phenomenon is no simple one. When we add the variety of circumstances affecting the action of different causes-as the latitude of the place, the elevation above the sea-level; the proximity of the sea; the laws affecting the seasonal variations at the place; the prevailing winds; and the configuration of the surrounding surface, it will become evident that meteorologists may well be perplexed by the very complex set of agencies acting in the production of rain; and so fail-as they have hitherto done-in interpreting any save the most general laws influencing the phenomenon. |